1. Field of the Invention
This invention relates to a method for fabricating a III nitride film, a substrate for epitaxial growth and a III nitride film, particularly usable for fabricating a semiconductor film constituting a semiconductor element, a substrate for the semiconductor element and the semiconductor film of the semiconductor element. Also, this invention relates to an epitaxial substrate for semiconductor element and a III nitride element.
2. Related Art Statement
III nitride films are used as semiconductor films for light-emitting diodes and electronic devices, and recently get attention as semiconductor films for high speed IC chips. Particularly, Al-including III nitride films get attention as field emitter materials.
Such an Al-including III nitride film is formed on a base made of sapphire single crystal or the like via a buffer layer made of III nitride and an underfilm made of Al-including nitride film as occasion demands. The buffer layer and the underfilm are made on the base in turn. Various functions are imparted to the III nitride film. Generally, the base, the buffer layer and the underfilm are regarded as an integrated substrate, which is called as a substrate for paraxial growth. Then, a single III nitride film or plural III nitride film are formed on the epitaxial growth substrate to fabricate a multilayered structure, which is called as an epitaxial growth substrate for semiconductor element.
The buffer layer is made at a low temperature within 500-700xc2x0 C. under low crystallinity so as to compensate the difference in lattice constant between the base and the underfilm by means of CVD or MBE and so on.
Since the buffer layer is made at such a low temperature, however, the crystallinity of the buffer layer is not sufficient, so that the crystallinities of the underfilm and the III nitride film which are formed on the buffer layer can not be enhanced sufficiently. In addition, the buffer layer includes much dislocations, so that the underfilm and the III nitride film also includes much dislocations due to the dislocation propagation from the buffer layer, and the mosaic properties of the films which can be evaluated from their X-ray rocking curves are enlarged. Moreover, the crystal quality of the III nitride film such as dislocation density and crystallinity are fluctuated largely by the thermal hysteresis at the formation of the buffer layer and the III nitride film.
As a result, the crystal quality such as crystallinity and dislocation density of the III nitride film to which various function are imparted can not be enhanced sufficiently, so that the performance of a semiconductor element including the III nitride film may be deteriorated. For example, with a semi-conductor light-emitting element, the luminous efficiency may be deteriorated.
It is an object of the present invention to provide a new method for fabricating a III nitride film whereby a high crystal quality Al-including III nitride film can be formed on a given single crystal base, and to provide a new substrate for epitaxial growth whereby the high crystal quality Al-including III nitride film can be formed. It is also an object of the present invention to provide the high crystal quality Al-including III nitride film which is preferably usable for a semiconductor element.
Moreover, it is an object of the present invention to provide a semi-conductor element and an epitaxial growth substrate for semiconductor element which includes the above high crystal quality Al-including III nitride film.
In order to achieve the above object, this invention relates to a method for fabricating a III nitride film, comprising the steps of:
preparing a base made of single crystal,
forming an acicular structure made of AlN on a main surface of the base, and
forming an Al-including III nitride film on the main surface of the base via the acicular structure.
This invention also relates to a substrate for epitaxial growth comprising:
a base made of single crystal, and
an acicular structure made of AlN and formed on a main surface of the base.
The inventors had intensely studied so as to form a high crystal quality Al-including III nitride film on a single crystal base. Then, the inventors focused attention on the condition of the main surface of the base on which the III nitride film is formed. As a result, if an acicular structure made of AlN is formed on the main surface of the base, and then, a given Al-including III nitride film is formed on the base via the acicular structure, without a buffer layer, the crystallinity of the III nitride film can be enhanced and the dislocation density of the III nitride film can be reduced.
In other words, with the fabricating method and the epitaxial growth substrate of the present invention, such a high crystal quality Al-including III nitride film can be fabricated. Concretely, the full width at half maximum (FWHM) at (002) reflection in X-ray rocking curve can be reduced to 100 seconds or below, and the FWHM at (102) reflection in X-ray rocking curve can be reduced to 2000 seconds or below, particularly 1500 seconds or below. Then, the dislocation density can be reduced 5xc3x971010/cm2 or below, particularly 1xc3x971010/cm2 or below.
In an additional III nitride film is formed on the above Al-including nitride film, the dislocation density of the additional III nitride film can be reduced to 1xc3x97109/cm2 or below, particularly 1xc3x97108/cm2 or below. The dislocation density reduction becomes conspicuous if the difference in Al content between the lower original III nitride film and the additional III nitride film is set to 0.5 or over, particularly 0.8 or over at mole ratio. Then, the dislocation density reduction becomes maximum if the additional III nitride film does not include Al element.
Accordingly, if the Al-including III nitride film such as above original III nitride film or the additional III nitride film is employed for a semiconductor element, the performance of the semiconductor element can be enhanced.